This happens to me more often than I'd like (and yet, still not "frequently")... sometimes Molly will be sleeping, and when she gets up, there will be a little blood, but we won't be able to find ANY on her and we're never able to figure out where it came from or why.
It usually worries me just because she's an older dog, so most things tend to worry me... but it's a little comforting to know I'm not the only one finding mystery blood!